This invention relates to sputter depositing of thin films, such as for thin film magnetic recording discs, and to methods and apparatus for manufacturing such discs. More particularly, the invention relates to a composite source target and method for fabricating by RF diode sputtering a thin film magnetic disc having a magnetic layer comprising at least two materials, such as platinum and cobalt and having a radial coercivity gradient.
As disclosed in the co-pending application of James E. Opfer and Bangalore R. Natarajan entitled "Thin Film Magnetic Recording Media" filed Nov. 3, 1983, Ser. No. 549,331, now U.S. Pat. No. 4,610,911 and assigned to the instant assignee, the magnetic properties of a thin film magnetic recording disc having a cobalt-platinum magnetic layer with selectively desirable properties may be established in accordance with the thickness of an underlying chromium film, the thickness of the cobalt-platinum film, and the concentration of platinum in the cobalt-platinum film.
It is known that the chemical composition of a sputtered film is usually the same as that of the cathode (target) from which it is sputtered. See "Handbook of Thin Film Technology" edited by L. I. Maissel and R. Glang and published by McGraw-Hill Book Co., New York, N.Y. (1970) at pages 3-28 and 4-39. Thus, to sputter, for example, a cobalt-platinum film containing about three percent platinum, the target would be a homogeneous composition of cobalt and platinum with the concentration of platinum in the target being about three percent. Alternately, on page 3-29 of this text, it is suggested that sputtered compositional or alloy films can also be obtained by the use of multiple targets, each of a single material, and that a wide range of compositions can be obtained by independently varying the sputtering rates of the targets.
There are, thus, two suggested known prior techniques for providing sputtered compositional or alloy films: utilization of a homogeneous target of the materials, or utilization of two or more independent targets, there being a separate target for each material.
The first of these known techniques is limited and non-variable in that the percentage composition of the various materials in the target is fixed and determines the composition of the sputtered film. Furthermore, in the case of cobalt and platinum, the cost of such a homogeneous target is prohibitive. The second known technique, while permitting some versatility in the composition of the sputtered film, by varying the targets, is also costly. Moreover, problems are encountered in this second approach when it is desired to simultaneously deposit films on opposite sides of a disc or other substrate. For example, to sputter a composition of two materials on the two sides of such a disc, at least four targets are required. That is, one set of two targets are required at each side of the disc for sputtering thereon. One target of each set being of a first material and the other target of each set being of a second material. In addition, each of the four targets would have to be provided with a separate power supply system if each were to be independently controllable. Independent control is necessary in order to vary the sputtering rate from each target, and thus the percentage composition of each material. Furthermore, an arrangement of multiple, discrete targets does not lend itself to providing discs with magnetic films of radially-varying coercivity. As explained below, this latter characteristic is particularly desirable for high density recording on magnetic recording discs.
When discs are used in a typical magnetic recording disc drive application, the disc is annular, is rotated, and a read-write head is positioned to fly over the disc so as to read or write on concentric tracks on the disc. The speed of travel of the head, relative to the disc, is greater and the head flies higher over the disc when the head is reading or writing onto outer tracks at outer diameters of the disc in comparison to inner tracks at inner diameters. If the write frequency is held constant, the recording density is much higher on tracks closer to the inner diameter of the disc in comparison to the density on tracks toward the outer diameter.
Assuming a disc has a magnetic layer with a constant radial coercivity, such as understood to be provided by the above described known techniques, writing on tracks near the outer diameter of the disc is impossible or unreliable unless the writing current is increased at such outer diameter tracks. Increased writing current is required because, as explained above, the head flies higher above the disc surface as the head moves outwardly from inner to outer tracks of the disc. In order to write with a constant current, which in many applications is highly desirable, the radial coercivity of the magnetic layer on the disc must be adjusted so as to decrease as the flying height of the head increases. In other words, the coercivity of the disc should decrease with increasing radial distances from the center of the disc.
The above described prior techniques simply do not have or suggest the provision of a magnetic layer with a radial coercivity gradient.
Therefore, a need exists for an improved sputtering method and target for sputtering a layer of composite materials onto a substrate. Moreover, a need exists for an improved sputtering method and target for providing thin film magnetic discs with a radial coercivity gradient.